Abstract
Intrinsic water use efficiency (WUEi) in trees is an indication of the ratio of carbon assimilation to the rate of transpiration. It is generally assumed that it is a response to water availability. In agricultural research, the question of drought tolerance by increased WUEi has been well studied. In general, the increase is a trade-off for productivity and is therefore not desired. For forest trees, this question is less clearly understood. Using stable carbon isotopes derived from tree rings combined with productivity as the product of the annual growth increment and annual density measurements, we compared the change in WUEi over a 15 year period. While WUEi increased over this period, the productivity decreased, causing an opposing trend. The gradient of the correlation between WUEi and productivity varies between provenances and sites. Counterintuitively, the populations at the drier site showed low WUEi values at the beginning of the investigation. Slopes vary with the provenance from Poland showing the least decline in productivity. In general, we found that a decline in productivity aligned with an increase in WUEi.
Highlights
Foresters have a major interest in the annual growth of trees [1], which is believed to increase due to globally rising CO2 concentrations [2]
In order to understand the difference in plasticity linked to individual and phenotypic responses, we investigated the changes in water use efficiency (WUEi) of three Norway spruce (Picea abies [L.] Karst) provenances at two sites in Saxony, Germany over a 15-year period and link it to annual stem growth increments as well as annual mean density measurements
We show that WUEi can be used as an indicator for stress experienced by mature trees
Summary
Foresters have a major interest in the annual growth of trees [1], which is believed to increase due to globally rising CO2 concentrations [2] This might be due to changes in either the photosynthesis rate or a lowered demand for water [3], explained by a CO2 fertilisation effect [4]. The link between an increase of CO2 , WUE, and productivity, while undoubtedly important with respect to growth and carbon sequestration models, is not fully conclusive [1]. This is likely due to the interaction of the effects of water availability, temperature [10], and tree age [11]. While there has been a decrease in, especially sulphur, deposition since the 1970s in most parts of Europe [13], values derived from the intensive forest monitoring still reach values between 8 and 16 kg/ha (mean of the years 2012–2014; [14]) in areas of central Europe
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